Problems with ECDSA signature and verification

[Matt Caswell]

On 17/10/2022 09:34, Fernando Elena Benavente wrote:
> Hi guys, we are having problems with the implementation of the signature 
> and verification of messages with ECDSA, because the demo of ECDSA in 
> github  us does not allow us to determine the type of ECDSA curve,

I assume you are looking at this demo:

https://github.com/openssl/openssl/blob/master/demos/signature/EVP_Signature_demo.c

The curve in use is a property of the key. So if you want to use a 
different curve then you need to generate a key for use with that 
different curve, e.g. for a key using the P-256 curve you can generate a 
PEM format one from the command line like this:

$ openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 -out 
privkey.pem

Or a DER format one like this:

$ openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 -out 
privkey.der -outform DER

To do this in C code you can just use the simple one liner:

EVP_PKEY *pkey = EVP_EC_gen("P-256");

Also see this demo code if your keygen requirements are more complex 
than just simply specifying the curvename:

https://github.com/openssl/openssl/blob/master/demos/pkey/EVP_PKEY_EC_keygen.c

> In 
> addition, we have seen that we have problems when it comes to having 
> strings and EVP_PKEY and not being able to pass one to another and vice 
> versa.

Your question here is lacking detail. It's unclear what you are trying 
to do, what you expected to happen and what actually happens.


> We are also not able to print EVP_PKEY keys because the BIO 
> functions in our version (3.0) are deprecated.

See the functions here:

https://www.openssl.org/docs/man3.0/man3/EVP_PKEY_print_public.html


Matt


> If you know the functions 
> to make this signature and verification from strings or even another 
> ECDSA example, would be great help for us.
> 
> Thanks for your help.
> 
> -Fernando
>